This invention relates to an apparatus and method for the measurement of the resistivity of concrete and cement-based materials having improved accuracy and repeatability in comparison with conventional technologies.
Concrete and similar cement-based materials are widely used in the construction and building industries because they are highly versatile low-cost building materials. In many applications it is necessary or desirable to measure the electrical resistivity of concrete or other cement-based materials.
Such measurements may be carried out, for example, in determining the physical properties and characteristics of such materials, especially with regard to monitoring how such properties change with the age of the material. In addition, in many situations concrete and cement-based materials may be used in the manufacture of buildings and rooms used for the research, manufacture and storage or the like of electrical components, and semi-conductor products. In many such cases it is desirable to form an electrically shielded room and if concrete or a like cement-based material is the primary construction material, then it is necessary to be able to determine the resistivity of the concrete or the like.
Conventionally, the resistivity of a concrete or cement-based material is determined by forming a specimen of the material in the form of a prism, and then measuring the current and voltage between two plate electrodes placed against opposing sides of the specimen. It will be readily apparent that in such conventional methods of resistivity measurement, the contact between the plate electrodes and the sample is of the greatest importance. If the contact is loose, inaccurate and sometimes nonsensical measurements will be obtained.
In order to improve the contact fresh cement paste or colloidal graphite can be placed between the sides of the specimen and the electrodes. Also, an external force can be used to clamp the electrodes firmly to the sides of the sample. These techniques, however, are only effective at the beginning of measurements, as hydration of the sample will cause shrinkage leading to the formation of cracks and fissures in the sample that eventually mean that the measurements can no longer be made with any reliability at all. Furthermore when a cement paste is used to improve the connection, because the paste is highly alkaline it corrodes the electrodes which thus have to be replaced at very regular intervals.
In summary there exists no completely satisfactory method of obtaining reliable, and repeatable measurements of the resistivity of a concrete or cement-based material.
According to the present invention there is provided apparatus for measuring the resistivity of a sample of cement-based material, comprising: a transformer formed with a primary coil, a secondary coil formed by means of a sample of said cement-based material, means for supplying a voltage to said primary coil, means for measuring the voltage induced in said sample, and means for measuring the current induced in said sample.
In a first embodiment the transformer is a double arm transformer and the primary and secondary are both formed around a transformer core.
In a second embodiment the transformer is a single arm transformer and the primary is wound around a transformer core and the secondary is formed around said single arm.
Preferably the sample is formed as a ring of said cement-based material. The ring may be annular, or may be rectangular.
Preferably the voltage measuring means comprises a coil formed on a surface of the ring and extending circumferentially around said sample. Preferably the current measuring means surrounds a radial section of said sample. The current sensing means may be a leakage current meter, or may be a Rogowski coil.
Viewed from another aspect the present invention provides a method for measuring the resistivity of a sample of cement-based material, comprising:
(a) forming a sample of said material as a secondary of a transformer,
(b) applying a voltage to a primary of said transformer,
(c) measuring the voltage induced in said sample,
(d) measuring the current induced in said sample, and
(e) determining the resistively of said sample from the results of steps (c) and (d).